Anti tie down palm buttons

Is there away to program palm buttons using
only one timer in the anti tie down ??

Maybe this will help.
Here

Or here

And here

Is this what your looking for?

Please note that this is a safety feature, and I'm not really comfortable using a PLC to address this issue.

Mike Williams said:

Maybe this will help.
Here

Or here

And here

Click to expand...

tks/m I had seen the logic with one timer but couldn't get it . Safety is of the upmost tks/again

Click to expand...

Click to expand...

Do NOT use a standard PLC for safety. In your country you are PERSONALLY liable in case of an accident !

There er safety relays for this kind of functions. Get one AND use it as prescribed in the documentation for the relay.

SAFETY, SAFETY, SAFETY

Do yourself a favor, and go out and buy a NOLATRON two-hand
safety relay, and call it a day.

Paul,

I have to agree with the other posts here advising not to use the PLC for this purpose. The code I posted is based on my understanding of what is required; the two buttons must be pressed and held within a certain time. However I have never had the opertunity to actually install this type of device.

The PLC should never be used to assure safety, it's just common sense. The purpose of my previous post was to answer you question, and not to advise you, or approve of, using a PLC for this purpose.

Mike.

You are learning Mike.

Most of our machines have light curtains, so I have been fine programming the two hand system. It helps prevent accidental starting of the cycle. It wouldn't be unsafe, just inconvenient.

I am curious how you would interface the nolatron unit into a system. It seems to me that if you just sent it into the plc, your benefit would be limited. How is this normally done?

I agree safety first, but I would like yall's thoughts about the safety relay. Since the safety relay is going to input to the plc after it gives the "OK TO START", what makes it safer than 2 seperate plc inputs. I mean, the plc makes the machine move anyway? Or is this just a liability (or legal) issue? Thanks..
mg

To make it very short...

The safety functions would not be wired through the PLC. They must be hardwired. Normally there are two output channels in the safety relay. Each channel then energizes a contactor. The two contactors are then wired in series so that when they are NOT energized they will break the power to the machine or process or...

All this to ensure that ONE error does not disable the safety.

There will often be one or more auxilliary contacts in the safety relay. Can be used to light a lamp, or tell the PLC that the relay is deactivated.

The lowest safety class (= not so severe, and very unlikely to happen) do not require two channels or two contactors.

Just out of courisoty, would the following hard wire be considered legal? Using my code above will monator the buttons and provide the output to run the press. However if the PLC output locks on the press still will not run unless both the buttons are pressed.

I understand the timer function would be lost in event of a PLC output lock-on, but still the press wont start without the buttons on.

                

                                  PLC

                                 OUTPUT

                                 COMMON 

                                   *     PLC

    LH BUTTON        RH BUTTON     |    OUTPUT        

-----*^*-------*-------*^*---------*-----||-----*-------(press)

               |                   |

               *                   *         

            PLC INPUT 1        PLC INPUT 2                      

Just to be clear, this is a question, not a suggestion.

Thanks,

Safety is always an issue BUT two hand start buttons are not always used on devices that can kill or maim. I recently completely redesigned an L sealer that had a moving arm with heat that cut and seal poly around a package. The original design used 2 button start but in no way had any safety features EXCEPT it kept the operators hands away from the hot wire cutting arm.

I investigated as thorough as possible and in that situation did not see a need or any regulations that required me to add more devices to the system....my goal was to use less components by replacing the relays with a plc. Overall when I was done the system was 5 times faster, the safeties redundant, less shock hazard, and IF someone did get there arm in it the response was so fast that they never got burned or bruised...which happened alot prior to that.

The point being is that just because 2 hand control is being used, it does not mean that the safety risk is great. A risk analysis would need to be done to determine whether or not it would be acceptable.

I would submit that two hand control only constitutes a small amount of safety. If any. What's to stop Joe's friend from sticking his hand in there? Plus it is so easily defeated.